1. Field of the Invention
This invention relates to a nonrinse type hydrophilic surface-treating process capable of imparting hydrophilicity to the surface of a metal material and, at the same time, forming thereon a coating excelling in resistance to corrosion (hereinafter referred to as "surface-treating process") and to the aluminum article treated with the surface-treating process.
2. Description of Prior Art
Metal materials, particularly aluminum metal and alloys, find extensive utility in numerous applications. In some applications, they are required to possess a surface that is readily wettable with water but is corrosion-resistant. In the case of a metal material used in fins for a heat exchanger, for example, an increasingly large number of functional requirements must be satisfied such as prevention of rusting, improvement is efficiency of energy consumption, and abatement of noise. The recent trend of heat exchangers toward improved performance combined with reduction in size has prompted a gradual reduction of fin interval or spacing. A heat exchanger relies on the surface of fins for exchange of heat between a heat or coolant medium circulated within and the ambient air without. When using the heat exchanger for space cooling, the moisture in the atmospheric air condenses on the surface of the fins. When the fin interval is reduced to less than 3 to 4 mm, the water condensate gathers into droplets and bridges the gaps between the adjacent fins. These water droplets offer increased resistance to currents of air, generate noise, impair efficient heat transfer and increase energy consumption. Consequently, it is desirable for the fins to possess a hydrophilic surface.
In addition to heat exchangers, the surfaces of other articles are desired to be protected against the formation of dew in a humid atmosphere, while other metal surfaces are required to remain glossy and, therefore, are expected to be free of clouding, and still other metal surfaces are desired to have a high affinity for water and aid in accelerating the evaporation of adhering water. To fulfil these requirements, the practice of providing hydrophilic coatings for such metal surfaces is common. The desirability of a more effective hydrophilic surface-treating process, therefore, is finding growing recognition.
Known methods available for providing hydrophilic surfaces for aluminum articles include, for example, (1) a process which resorts to treatment with beohmite, (2) a process which relies on treatment with chrome-phosphate claimed to produce a chromate coating of relatively high hydrophilicity than other forms of chromate, and (3) a process which consists in applying a hydrophiic paint. As shown in Table 1, these methods have defects of their own and cannot be considered an adequate solution.
TABLE 1 __________________________________________________________________________ Treatment with Treatment chrome- Hydrophilic paint with boehmite phosphate Surfactant type Silica type __________________________________________________________________________ Wettability Initial stage of use O O O O after treatment Durability in O X X O protracted service Corrosion Test by exposure to X O X .DELTA. resistance moisture Press moldability (drawless type) .DELTA. O O X Problems Prolonged Waste chromate extra step for undercoating treatment solution is required for ensuring required. threatens water desired adhesion of paint. Bath control pollution. is difficult. Poor corrosion resistance. __________________________________________________________________________ Note: In the table, the test results are rated by the threepoint scale, wherein O denotes satisfactory performance, .DELTA. partially inferior performance, and X totally inferior performance. The same scale is used i Table 2 to follow.
Another known process for hydrophilic treatment uses a chromate bath incorporating therein a water-soluble type polyacrylic acid resin. The coating formed by this process is intended as an undercoat and, therefore, is deficient in hydrophilicity and corrosion-resistance. If to enable this coating to be used as a regular surface coating, either of these known processes were modified to incorporate such generally known expedient as adding silica powder to the bath thereby enhancing the hydrophilicity of the resultant coating or increasing the hexavalent chromium ion concentration in the chromate bath thereby imparting improved corrosion resistance to the produced coating, the problem still would not be solved. When the silica powder is added to the conventional bath, it must be added in a generous amount to obtain the desired enhancement of the hydrophilicity. As the result, the density or coherency of the inorganic coating is impaired and the stability of the coating to resist corrosion is degraded. If the hexavalent chromium ion concentration is raised as a compensatory measure, then there ensues a disadvantage that the produced coating while in use exudes excess hexavalent chromium ion. Thus, the desired improvement of the coating properties cannot be obtained by simply applying the aforementioned means to the conventional bath used in its unaltered form.